Along with the recent progress of information technologies, general purpose computer systems with advanced and powerful computing capabilities have been widely used in the offices of various research organizations and companies as well as in common families. The applicable fields of computers have also extended, wherein not only computer data but also a variety of media data such as images (including both static and moving images) and audio have been treated on computers as digitized files.
For example, in the fields of medical and diagnostic technologies, patients are diagnosed their symptoms based on fluoroscopic images or cross-sectional images of their bodies, which are taken by various modality apparatuses such as a CT (Computed Tomography) apparatus, MR (Magnetic Resonance) apparatus, and CR (Computed Radiography) apparatus.
Conventionally, diagnostic images of patient bodies taken by these kinds of modality apparatuses have only been directly printed out to sensitive films by an image output apparatus that is installed near the modality apparatus. Accordingly, managing the diagnostic images afterward is performed by manually sorting and arranging the films as physical media, thus it requires a significant man power to move, distribute and share the diagnostic images, thereby leading to inefficient works.
On the contrary, recently diagnostic images taken by modality apparatuses are digitized by a reader and managed by a computer as image files. Furthermore, by interconnecting modality apparatuses in a hospital and computers used by doctors and nurses via a network laid in the hospital, medial and diagnostic information such as diagnostic images and medical charts can be treated transparently in the network space. Namely, it becomes possible to transfer the diagnostic images to a remote terminal or to share the medical and diagnostic information among each of the terminals or to perform cooperative diagnoses on the network.
For example, diagnostic image files taken by a radiographer using a modality apparatus and verified afterward are once stored in an image storage server on a network. In this case, a doctor who needs a diagnostic image, can access the image storage server from a terminal on his desk (or in a consultation room), retrieve the diagnostic image, and further transfer the diagnostic image along with the diagnostic results to the image storage server, whereby the diagnostic data of a number of patients can be managed in a lump in the hospital. Moreover, a doctor can retrieve later the diagnostic image from the image storage server and determine a healing condition in time series by comparing it with the latest one. Medical records such as diagnostic images and diagnostic results are obliged or recommended to be saved at medical institutions for a given period of time.
In addition, providing a print server on the network allows sharing of an expensive printer for film printing among multiple modality apparatuses. Namely, technicians and doctors can transfer the image files taken by the modality apparatuses or image files stored on the image storage server to the remote print server to print them out on the films.
Furthermore, it is also possible to install on the network the workstations (WS) for viewing images, that is, image viewers, in addition to the modality apparatuses. Doctors can interpret and diagnose the diagnostic images taken by the modality apparatuses on the image viewer. For interpretation, the image processing conditions applied to the diagnostic images may be changed or corrected by the doctor.
By the way, in a hospital (in particular a large-scaled general hospital), a number of modality apparatuses are installed as sources of diagnostic images. Wherein on each of the modality apparatus, technicians who operates the apparatuses (e.g., radiographers) may take photographs of affected parts or entire bodies of patients or investigate the taken images and then send image data to the image storage server one after another. Doctors retrieve diagnostic image data from the image storage server via the network for interpretation and diagnosis, thereafter they store it in the image storage server as image files with diagnostic results appended. In other words, the image storage server stores a large number of image files to which plural file operators (e.g., doctors or radiographers) perform various operations of their own accord.
The image storage server generally comprises a huge hard disk drive, which temporarily stores vast amounts of image files sent from modality apparatuses or computers used by doctors. However, as a result of endless medical practices, the total capacity of the image files to be retained as medical records may exceed the storage capacity of the hard disk drive at a relatively early stage.
In view of this, the image files temporarily stored on the hard disk drive are moved to removable media such as a DVD (Digital Versatile Disc) or MO (Magnetic-Optical disc), in order to permanently retain the medical records. Though one removable medium naturally has its limit of storage capacity, a nearly inexhaustible amount of image files can be permanently stored by exchanging a medium loaded in the media drive.
By the way, a doctor can perform interpretation and diagnosis by outputting diagnostic images taken by the modality apparatus on a high-resolution CRT display connected to the image viewer. For interpretation, the conditions for image reading or for image processing that are applied to the diagnostic images on the image viewer may be changed.
When displaying such diagnostic images, it is desirable to reproduce the diagnostic image within a range of luminance which human beings are interested in, in order to perform a correct interpretation. However, depending on the difference of characteristics of luminance logarithmic values of the image display devices, even the images with the same dark and light values may appear differently on the respective image display devices. Suppose, for example, that after a radiographer performs quality assurance (QA) for diagnostic images on some display after taking some radiographs, a doctor interprets the diagnostic images on another display. In this case, the doctor may see the images that appear differently from those at the time of quality assurance. In this case, the quality assurance by the radiographer has little meaning and the doctor can not perform a correct interpretation, which may lead to misdiagnosis such as an oversight of the affected areas.
In general, it is preferable that the luminance logarithmic values change uniformly according to the dark and light values so that the appearance in the eyes of observers, that is, the spectral luminous efficacy is constant in any luminance area. On this account, the gradation table, which is the correction curve to give linearity to the luminance logarithmic values, is provided for the image display devices in order to display images with correcting the image values using the gradation table. For a CRT display, for example, since the spectral luminous efficacy decreases at the dark areas, the gradation table has a gradation correction curve to maintain linearity in such luminance areas.
However, this gradation correction curve has a strong dependence on hardware so that it differs depending on the types of display devices. Furthermore, even the same type of displays have unique properties. Also, even the same display device has a correction curve that changes with time. For example, the maximum luminance of CRT (Cathode Ray Tube) display decreases with aged deterioration.